1. Field of the Invention
The present invention relates to a photomask employed in the lithographic process when semiconductor devices or the like are manufactured and a method of manufacturing a photomask.
2. Description of the Related Art
Generally, the lithographic process employs photomasks having the portions which are transparent to an illumination light and those which do not allow light to pass therethrough, both portions forming a predetermined transfer pattern. Such a photomask is projected on a layer of photosensitive material formed on a substrate by a lens system, by which the pattern on the photomask is transferred to the substrate.
FIG. 1A is a section of a conventional photomask. A light blocking member 2, made of Cr or MoSi, is formed on the surface of a transparent substrate 1 made of glass or the like. The light blocking member 2 forms a pattern to be transferred.
The photomask of the above-described type is conventionally manufactured in the manner described below: First, a thin film 3 of Cr is formed on a transparent substrate 1, and an electron beam resist layer 4 is then formed on the thin film 3 of Cr, as shown in FIG. 2A. Next, a pattern transfer is conducted on the electron beam resist layer and the electron beam resist layer 4 is thereby patterned, as shown in FIG. 2B, by drawing a predetermined pattern on the electron beam resist layer 4 with an electron beam 5 and by developing the electron beam resist layer 4. Thereafter, the thin Cr film 3 is etched using the pattern of the electron beam resist layer 4 as a mask to obtain a pattern of light blocking member 2, as shown in FIG. 2C. Finally, the electron beam resist layer 4 is stripped, as shown in FIG. 2D.
In an image of the photomask manufactured in the manner described above which is projected onto the substrate, the light which passes through the transparent substrate 1 bends around the light blocking member 2 due to diffraction, as shown by the amplitude distribution graph of FIG. 1B. The intensity of light obtained in an actual operation is the square of the amplitude, and the light arrives under the light blocking member 2, as shown in FIG. 1C and, reducing the resolution of the pattern transferred and making transfer of a fine pattern with a high degree of accuracy difficult.
Reduction in the resolution due to diffraction is prevented by the phase shifting method. In this method, in the case of a photomask on which transparent portions T1, T2 . . . and light blocking portions S1, S2, S3 . . . are alternately disposed, a phase member 13 is disposed on every other transparent portion. More specifically, in the transparent portion T2, the phase member 13 is formed on the transparent substrate 11 between the adjacent light blocking members 12, as shown in FIG. 3. The phase member 13 has a thickness which ensures that the phase of the light which passes through the phase member shifts by 180.degree. from that of the light which does not pass through it.
Hence, the light which passes through the transparent portion T1 and bends around the light blocking portion S2 and the light which passes through the transparent portion T2 and arrives under the light blocking portion S2 interfere with each other and thereby cancel each other, resulting in improvement in the resolution.
The photomask shown in FIG. 3 is manufactured in the manner described below. First, a light blocking member 12 having a predetermined pattern is formed on a transparent substrate 11 in the same manner as that shown in FIGS. 2A to 2D, as shown in FIG. 4A. Next, a transparent film 14 is formed on the transparent substrate 11 and on the light blocking member 12, as shown in FIG. 4B. Thereafter, a resist layer is formed on the transparent film 14, and a pattern transfer is then conducted on the resist layer and a resist pattern 15 is thereby formed by drawing a pattern on the resist layer with an electron beam or the like and by developing the resist layer, as shown in FIG. 4C. In this resist pattern 15, the resist layer is left behind alternately on the transparent portions where no light blocking member 12 is provided. Thereafter, the transparent film 14 is etched using the resist pattern 15 as a mask to form a pattern of phase members 13 shown in FIG. 4D. Finally, the resist pattern 15 is stripped.
However, in the photomask shown in FIG. 3, since interference of the light which passes through the transparent portion T2 where the phase member 13 is disposed and of the light which passes through the transparent portion T1 located adjacent to the transparent portion T2 is utilized, the phase shifting method is adopted only for a repetitive pattern in which the transparent portions and the opaque portions are alternately formed.
Furthermore, manufacture of the photomask shown in FIG. 3 includes two pattern transfer processes, one for patterning the resist layer 4 shown in FIG. 2A and the other for forming the resist pattern 15 shown in FIG. 4C. In consequence, the manufacturing method is made complicated, and generation of defective patterns is facilitated. Furthermore, a highly accurate registration of the pattern is required.